Accurate and reliable measurement of the angular distance, the angular velocity, and the rotating direction of a rotatable shaft attached to a motor or a gear is frequently needed for various purposes such as the control of precision instruments. To meet such a requirement, several hardware constructions have been proposed resorting to digital control. One example of those constructions is the optical rotary encoder comprising a glass disk with a plurality of photo-slits, and at least one pair of an LED (light emitting diode) and a photo-sensitive diode which are positioned at opposite sides of the glass disk. However, this optical encoder has encountered some problems: (1) it requires a highly accurate exposure as well as a high etching technique to provide the photo-slits on the entire surface of the glass disk without aberration, (2) it is practically very difficult to accurately adjust the center of the photo-slits with that of a bore receiving a rotatable shaft, (3) in order to snugly receive the rotatable shaft, such a bore should be precisely processed and finished, which causes some practical difficulty, (4) as a consequence the encoder becomes costly for achieving initially intended results, and (5) furthermore, the encoder is vulnerable to external shocks.
As another approach to attain the above-mentioned purposes, a magnetic tachometer has been proposed by A. B. Wills in IBM Technical Disclosure Bulletin, Volume 16, No. 1 (June issue, 1973), page 260. The tachometer comprises a disk carrying a magnetic medium on one surface near its perimeter, and a magnetic flux transducer. The transducer senses magnetic signals recorded on the magnetic medium to determine the rotational speed of the disk, namely, the rotational speed of the shaft to which the disk is connected. However, the Bulletin does not disclose a magnetic tachometer in a concrete manner but only its possibility.
As still another approach, an example of a transducer apparatus for rotary displacement is described in U.S. Pat. No. 4,039,936. According to this patent, the apparatus comprises two transducers each having a pair of U-shaped magneto-resistors mounted on a permanent magnet. The magnet is supported in a fixed position adjacent a rotatable disk which has teeth and is moved in either direction. Although the transducer is resistive to external shocks is easy manufacture as opposed to the above-mentioned optical rotary encoder, the pitch of the teeth, which determines the accuracy of rotary displacement, cannot be made so small as to precisely measure the rotary displacement.
It is therefore one object of this invention to provide a magnetic encoder for accurately and reliably detecting incremental angular displacement as well as angular velocity with relatively simple construction.
It is another object of this invention to provide a magnetic encoder for accurately and reliably detecting incremental angular displacement, angular velocity, and the rotating direction of a shaft connected to the encoder with relatively simple construction.